Keys to successful grain sorghum

Everything you always wanted to know about growing grain sorghum in Arizona but were afraid to ask is found in a new research publication authored by the University of Arizona’s (UA) Cooperative Extension agronomist Mike Ottman and plant pathologist specialist Mary Olsen.

Grain sorghum or milo is a warm season, annual grain crop that is more resistant to salt, drought, and heat stress than many other crops. The highest yields are achieved when stresses are minimized.

• Hybrids

Grain sorghum hybrids are classified as short, medium, or full season, according to Ottman and Olsen. Medium and medium-full season hybrids are generally grown in Arizona. Hybrids grown for grain usually do not exceed 4 feet in height. Dual-purpose hybrids utilized for forage and grain can exceed 5 feet.

Grain color can be purple, red, brown, bronze, tan, yellow, creamy, or white. Bird damage to the heads may be reduced in hybrids with high tannin content in the grain.

• Planting date

Sorghum seed will germinate when the soil temperature at the seeding depth is 50 degrees F. Faster germination and superior establishment is obtained when the soil temperature is 60 degrees F., at 8:00 a.m. for more than five consecutive days. Sorghum can be planted in the summer, but grain yields are usually less than a spring planting.

The optimum summer date to plant is late enough to avoid the heat during bloom, but early enough to avoid frost and poor fall drying conditions.

• Planting configuration

Grain sorghum can be planted on beds or on flat ground. Sorghum row spacing ranges from 6 to 40 inches apart or in twin rows in a variety of configurations. The UA report says higher yields will result from plantings in 20 or 30-inch rows compared to 40-inch rows or in twin rows spaced 12 to 14 inches apart on 38 to 40-inch beds.

• Seeding rate

The optimum seeding rate for grain sorghum is about 10 pounds of seed per acre assuming a seed size of 14,000 seeds per pound and 70 percent emergence. Seed size varies from about 13,000 to 16,000 seeds/pound depending on the hybrid. The seeding rate on a pound-per-acre basis should be decreased for smaller seed and increased for larger seed.

The goal is to achieve 100,000 plants per acre.

• Seeding depth

The optimum seeding depth is 1 to 2 inches, according to the researchers. Seed deeper on lighter soils or if seeding into moisture rather than irrigating up.

• Fertilizer

At the 6,000 pound/acre yield level, the plant will take up about 150 pounds of nitrogen per acre. The actual requirement depends on grain yield, the soil nitrogen level, irrigation water, and other factors.

Phosphorus fertilizer is generally not required except when planting in the cooler part of the spring and the soil phosphorus level is less than five parts per million. Potassium fertilizer is not generally used in most Arizona soils. Iron deficiency can occur on highly alkaline soils. Zinc deficiency can occur where topsoil has been removed by land leveling.

• Irrigation

Sorghum requires adequate soil water for maximum yields even though the crop is drought tolerant. The amount of water used by sorghum in a late June planting in the Phoenix area was measured at 25 inches. Assuming an irrigation efficiency of 70 percent, the actual irrigation water needed to meet this water use is 36 inches.

Daily water use is relatively low during establishment, peaks at about .4 inches per day at bloom, and then steadily declines. Some soil moisture is necessary at grain maturation to maintain stalk integrity.

When using border flood irrigation on a sandy loam soil, about six irrigations of 6 inches each are required, the report says. The actual number of irrigations will vary depending on soil type, hybrid maturity, and rainfall.

• Weeds

Weeds can reduce yield, harbor insects and diseases, and create problems at harvest. Weeds are controlled by tillage or herbicides. While sorghum eventually establishes a dense canopy, it is not particularly competitive with weeds early in the season due to slow crop development.

Controlling grass weeds in sorghum can be problematic due to the limited effectiveness of herbicides for post-emergence applications. A pre-plant herbicide application is best for grass control, or the use of seed safeners which allow the application of more effective post-emergence herbicides.

Many broadleaf weeds can be controlled with tillage or herbicides, or the crop can out-compete these weeds. The one exception is morninglory which can grow over the crop creating problems at harvest. The weed should not be tolerated.

Chemical treatment is not warranted for most insects unless the damage is severe. Severe insect damage is most common when sorghum is planted late or following a corn or sorghum crop. Leaf feeders including the corn earworm and fall armyworm generally do not cause economic damage.

The southwestern corn borer can cause damage and crop monitoring is recommended. The borer feeds in the leaf whorls causing a shot-hole appearance to the leaves when they unfold. After feeding on the leaves, the larvae move down the plant and bore into the stalk where the real damage occurs. Tunneling in the stalk especially by the second and succeeding generations weakens the stalk and may cause lodging.

Southwestern corn borer should be chemically controlled when 25 percent of the whorls are infested.

Cultural practices include: avoid planting after corn or sorghum; planting early-maturing hybrids; sowing the crop early; planting over a short period of time over an area-wide basis to avoid population build-up; maintaining strong stalks through proper plant population and irrigation and fertilizer practices; and surface tillage at harvest to break the stalks and expose the larvae to cool winter temperatures and desiccation.

• Diseases

Grain sorghum grown in Arizona is not significantly damaged by diseases. The few diseases found in Arizona include head smut, maize dwarf mosaic virus, fusarium root rot, and root-knot nematode.

• Harvest aid chemicals

Various chemicals used near harvest are designed to stop crop growth, desiccate foliage, or dry the grain. The products include sodium chlorate or herbicides including paraquat or glyphosate. The chemicals work best when the temperature is warm and the air is dry.

Harvest aid chemicals have not been tested in Arizona, but experience from other regions suggest that while crop growth may be stopped and foliage desiccated, rarely do these chemicals speed the grain drying process. Terminating irrigation about two to three weeks after bloom and avoiding excessive nitrogen rates may enhance the grain drying process.

• Harvest

When to harvest sorghum can be a difficult decision due to uneven maturity. Depending on the hybrid the stalks and leaves may still be green when the grain is ready for harvest. Sorghum can be harvested when grain moisture content is 18 percent to 23 percent. Harvest losses increase outside of this moisture range.

Grain cannot be stored safely over a long period of time above a moisture content of 12 percent to 14 percent. Artificial drying is necessary for high moisture grain. Unfortunately, artificial drying facilities are not available in Arizona, so grain should be harvested when its moisture content is below 14 percent.

Sorghum grain absorbs moisture from the atmosphere, and grain moisture content can change a few percentage points during the day.